P-xylene (PX) is one of basic organic feedstock's in petrochemical industry, and finds widespread use in the production of various chemicals such as chemical fibers, synthetic resins, agrochemicals and medicines. P-xylene is typically produced from an ethylbenzene-containing xylene stream, i.e., C8 aromatic hydrocarbon stream (C8A), in thermodynamic equilibrium derived from catalytic reforming of naphtha, wherein p-xylene is separated from a mixture of isomers having similar boiling points through a multi-stage cryogenic crystallization separation technique or a simulated moving bed molecular sieve adsorption separation (briefly referred to as adsorption separation) technique. The separated o- and m-xylenes are generally isomerized into p-xylene through a C8A isomerization (briefly referred to as isomerization) technique. Disproportionation of toluene or disproportionation and transalkylation of toluene and C9+ aromatic hydrocarbons (briefly referred to as toluene disproportionation and transalkylation) may be utilized to produce benzene and C8A, thereby obtaining even more p-xylene.
Until now, the relatively well-developed processes associated with toluene disproportionation include conventional Tatoray toluene disproportionation process as industrialized at the end of 1960's, MTDP process as put forward at the end of 1980's, and S-TDT process and TransPlus process as put forward in recent years. Toluene selective disproportionation is a new route for the production of p-xylene, wherein toluene undergoes selective disproportionation over a modified ZSM-5 catalyst to produce benzene and C8A with a high concentration of p-xylene, and a majority of p-xylene can be separated through only a simple step of freezing separation. In recent years, as the performance of the catalysts is continuously improved, this process is significantly developed. It is represented by MSTDP toluene selective disproportionation process as industrialized in the later stage of 1980's and pX-Plus process as put forward in recent years.
The MSTDP toluene selective disproportionation process comprises treating a toluene feedstock with a modified ZSM-5 mesoporous molecular sieve catalyst, to produce C8A with a high concentration of p-xylene (85-90%, by weight, the same below unless otherwise specified) and nitration grade benzene. The pX-Plus process, of which industrial application has not been reported, has the following main technical parameters: in the case of a toluene conversion of 30%, the selectivity of PX in xylenes reaches 90%, and the molar ratio of benzene to PX is 1.37.
However, such toluene selective disproportionation processes have a strict requirement on the selection of feedstock while having a high para-selectivity. These processes can only employ toluene as the feedstock, and C9+A cannot be used (at least cannot be directly used) in these processes, resulting in the waste of aromatic hydrocarbon resources. Furthermore, these processes produce a large quantity of benzene as by-product, resulting in a relatively low yield of p-xylene, which is a fatal shortcoming of the selective disproportionation processes.
Typical feed to a reactor in Tatoray process comprises toluene and C9 aromatic hydrocarbons (C9A). The xylene produced by Tatoray process is a mixture of isomers in thermodynamic equilibrium, wherein the content of p-xylene that is the most valuable in industry is generally only about 24%. It is undoubted that Tatoray process possesses an obvious disadvantage relative to the toluene selective disproportionation processes that can produce mixed xylenes having a p-xylene concentration of about 90%. However, relative to the toluene selective disproportionation processes, Tatoray process has a great advantage that it is capable of converting C9A into benzene and xylenes. Literatures relating to Tatoray process include, for example, U.S. Pat. No. 4,341,914, CN98110859.8, U.S. Pat. No. 2,795,629, U.S. Pat. No. 3,551,510, and CN97106719.8. A representative process as set forth in U.S. Pat. No. 4,341,914 comprises the steps of subjecting a reformed product to fractionation of aromatic hydrocarbons; feeding the resulting toluene and C9A to a Tatoray process unit for disproportionation and transalkylation; separating a reaction effluent; recycling toluene, C9A and a portion of C10 aromatic hydrocarbons (C10A), and collecting benzene as a product; passing C9 aromatic hydrocarbons together with additional C8 aromatic hydrocarbons from an isomerization unit to a PX separation unit to separate a high purity p-xylene product; and passing other C8 aromatic hydrocarbon isomers to the isomerization unit for isomerization of xylene, to obtain mixed xylenes in thermodynamic equilibrium again.
In recent years, as the rising of the toluene selective disproportionation processes, a process for the production of C6-C8 aromatic hydrocarbons by dealkylation of heavy aromatic hydrocarbons draws more and more attention. U.S. Pat. No. 5,763,721 and U.S. Pat. No. 5,847,256 respectively propose catalysts useful in the dealkylatoin of heavy aromatic hydrocarbons. Among these, U.S. Pat. No. 5,847,256 discloses a rhenium-containing mordenite catalyst, which is especially suitable for the conversion of feedstock enriched in aromatic hydrocarbons having one or more ethyl groups to form toluene, xylenes, benzene, etc.
The various C8 aromatic hydrocarbons have similar boiling points: 136.2° C. for ethylbenzene, 138.4° C. for p-xylene, 139.1° C. for m-xylene, and 144.4° C. for o-xylene. O-xylene having the highest boiling point can be separated by rectification process, which however requires more than one hundred of theoretical plates and a relatively great reflux ratio. Ethylbenzene having the lowest boiling point can also be separated by rectification process, which however is much more difficult. The various C8 aromatic hydrocarbons have markedly different melting points: 13.3° C. for p-xylene, −25.2° C. for o-xylene, −47.9° C. for m-xylene, and −94.95° C. for ethylbenzene. P-xylene has the highest melting point, and can be separated by crystallization process. If the concentration of p-xylene in the feedstock is not high, a two-stage crystallization process is generally employed in order to achieve an industrially acceptable yield. The process disclosed in U.S. Pat. No. 3,177,255 and U.S. Pat. No. 3,467,724 comprises the steps of: crystallizing most of p-xylene at a low temperature of −80 to −60° C., to achieve a yield close to the maximum theoretical yield, the crystal obtained having a purity of 65 to 85%; melting the crude xylene crystal followed by a second crystallization at a temperature of generally −20 to 0° C., to obtain p-xylene with a purity of above 99%; and recycling the mother liquor from the second crystallization, which has a relatively high p-xylene level, to the first crystallization stage.
By virtue of the difference in selectivity of an adsorbent for various C8 aromatic hydrocarbons, p-xylene can be separated by an adsorption separation process. This process has become one of the major processes for the production of p-xylene once it has been industrialized in 1970's. U.S. Pat. No. 2,985,589 describes a process for the separation of p-xylene by using a countercurrent simulated moving bed; U.S. Pat. No. 3,686,342, U.S. Pat. No. 3,734,974 and CN98810104.1 describe the use of Ba-type or Ba and K-type X or Y zeolite as an absorbent in adsorption separation; U.S. Pat. No. 3,558,732 and U.S. Pat. No. 3,686,342 respectively describe the use of toluene and p-diethylbenzene as a strippant in adsorption separation.